The present invention is directed to a velocity sensing system and, more particularly, to a velocity sensing system for a damper or the like.
A typical damper assembly, such as the one shown in FIG. 1, includes a damper body 12 having a piston (not shown) slidably disposed within the damper body 12. The damper assembly includes a piston rod 14 fixed to the piston and extending outwardly from the damper body 12. The damper assembly typically is associated with a spring and is mounted between a wheel assembly and frame or body of a vehicle, such as an automobile or a truck. The piston is located in a fluid-filled cavity (not shown) of the damper body 12. When a load, shock or vibrational force displaces the associated wheel assembly relative to the vehicle body, the force drives the piston into, or out of, the damper body 12; the movement of the piston through the relatively viscous fluid within the cavity dampens the movement of the wheel in a well-known manner.
The damper assembly also may include a dust tube 16 coaxially received over the damper body 12. The dust tube 16 provides mechanical protection to the damper body 12 and reduces the introduction of dust and other contaminants into the damper body 12. When the damper body 12 is moved relative to the piston and piston rod 14, the damper body 12 moves along its central axis relative to the dust tube 16.
It is often desired to track the state of the damper assemblies of a vehicle so that an on-board control unit may account for the state of such damper assemblies in controlling the damping characteristics of the damper assemblies, as well as during control of braking systems, steering systems and the like. In particular, the control of dampers in real time damping systems requires measurement of the instantaneous relative damper velocity (i.e., the velocity of the piston relative to the damper body 12 or the velocity of the damper body 12 relative to the dust tube 16) as a control variable.
FIG. 1 illustrates a first system, generally designated 10, for determining the velocity of the damper body 12 relative to the dust tube 16. The system 10 includes a concentrated magnet 19 (such as a ring magnet) mounted to the top of the damper body 12. The dust tube 16 includes a coil 20 located on and distributed along its inner surface, with the coil 20 being coaxial with the dust tube 16 and damper body 12. Movement of the damper body 12 relative to the dust tube 16 causes a voltage to be induced in the coil 20, which may then be sensed to determine the damper velocity.
The theoretical or idealized flux of the system 10 is shown as line 22 in FIG. 1. The flux 22 exits the magnet 19 in a radial direction and extends across a radial gap 24 to the dust tube 16. The flux 22 extends up the dust tube 16, radially across the top 26 of the dust tube 16 and axially along the piston rod 14 to return to the magnet 19 to close the flux loop 22.
The system 10 of FIG. 1 may provide adequate data when the stroke of the damper is relatively small (i.e., less than two times the diameter of the damper body 12). However, when the stroke of the damper is relative large (i.e., more than two times or four times the diameter of the damper body 12) the performance of the system 10 of FIG. 1 may be unacceptable. In particular, as the distance of the magnet 19 from the top 26 of the dust tube 16 (through which the flux 22 passes) increases, the size of the flux path 22 increases and flux loss increases accordingly, which degrades the performance of the system 10. Furthermore, the flux 22 of the system 10 of FIG. 1 is sensitive to disruption caused by the radial flux produced by magnetorheological (“MR”) fluid type dampers, which typically include solenoids located inside the damper body 12.
FIGS. 2A and 2B illustrate an alternative system, generally designated 30, for determining damper velocity. The system 30 includes two diametrically opposed magnets 32, 34 positioned adjacent to and extending axially along the length of the damper. Each magnet 32, 34 has a polarity such that the long, flat face 36 thereof facing the damper body has a north or south polarity and the opposite face 38 has an opposite polarity. The magnets 32, 34 are mounted such that the faces thereof facing the damper body 12 have opposite polarity. Each magnet 32, 34 is mounted on a flux carrier or flux collector 40 which is part of, or coupled to, an external dust tube (not shown). The system 30 of FIGS. 2A and 2B differs from the system in FIG. 1 in that the magnets 32, 34 are located on the dust tube instead of on the damper body 12 and the magnets 32, 34 are distributed or extend along the entire length of the damper travel rather than being concentrated at the end of the damper tube 12. Conversely, the coil 20 of the system of FIG. 1 that was distributed along the entire length of travel is now concentrated at the top of the dust tube as two separate coils 44, 46.
The idealized flux of the system 30 of FIGS. 2A and 2B is shown therein as flux path 48. The flux travels from the north pole or face of one of the magnets 34 and across the damper body 12 to the south pole of the other magnet 32 in a generally radial or circumferential direction. The flux 48 then travels axially along the flux collector 40 to the top 50 of the dust tube. The flux 48 then travels generally radially or circumferentially to the other flux collector 40 and returns to the north pole of the magnet 34 to close the flux loop 48. The sense coils 44,46 may be located on the flux collectors 40 (see FIG. 2A) or at the top 50 of the dust cover (see FIG. 2B) to sense the voltage generated by movement of the damper body 12.
The system 30 of FIGS. 2A and 2B can provide a reduced-quality output signal because the flux 48 is required to be carried axially to the top 50 of the dust tube to pass through the coils 44, 46. This requires a relatively long flux path 48 (especially during extended travel or long strokes of the damper body 12) that reduces the signal strength due to flux leakage.
Accordingly, there is a need for a velocity sensing system wherein flux leakage is reduced and signal strength is increased.